Gas discharge lamps such as fluorescent lamps are most efficiently operated when driven with an AC voltage of high frequency, typically 30 KHz. Such a drive voltage is typically generated by a resonant "tank" circuit made up of an inductive element and a capacitive element. The tank circuit is typically supplied from a utility mains (e.g., having voltage of 120 VAC, 60 Hz) via a rectifier and an inverter. The inverter typically includes series-connected transistors whose control electrodes are transformer-coupled to the tank circuit output so that the inverter transistors are alternately switched ON and OFF, providing to the tank circuit a supply which alternates at the frequency of the tank circuit.
Conventionally, in such a resonant circuit the control electrodes of the inverter transistors are coupled to the tank circuit output by saturating-core transformers. The use of saturating-core transformers ensures automatic cyclical switching from one of the inverter transistors to the other as the respective transformer core saturates. The use of saturating-core transformers enables rapid switching of the inverter transistors, allowing relatively tight control of the inverter output.
However, such saturating core transformers are highly specified components which are typically expensive. Also, the behavior and performance of such saturating core-transformers, in particular the point at which their saturation will cause switching of the inverter, is in practice highly dependent on their precise core characteristics and on their temperature, and is extremely difficult to predictably control. For example, it is common for two such nominally identical saturating-core transformers from different batches made by the same manufacturer by the same process to have significantly different saturation characteristics due to slight differences in the ways that the cores' materials were fired. It has been found that the saturation characteristics of such nominally identical saturating-core transformers may differ by as much as .+-.20%. The unpredictable saturation characteristics of such typical saturating-core transformers makes difficult the design and stable performance of high quality inverters using such saturating-core transformers and having predictable, well defined operating characteristics.
A further and particularly critical problem arising from the inclusion of such saturating-core transformers is that a saturating-core transformer has its own characteristic frequency at which it will naturally saturate. If this characteristic saturation frequency should become less than the operating frequency of the inverter, cross-conduction of the inverter transistors will occur during switching, destroying the transistors.